1. Field of the Invention
The present invention relates to systems for attitude control of Earth-orbiting satellites and, more particularly, to methods and systems for controlling spacecraft attitude.
2. Prior Art
Star trackers are the nominal non-inertial attitude reference system providing non-inertial attitude reference data. Other sensors, such as magnetometers and sun sensors also provide non-inertial attitude reference data. Star trackers are electro-optical devices that focus a field of view of a segment of the sky onto a detector that measures x-y positions of star images within the detection coordinates. The measured x-y positions of the stars are signal processed and identified by mapping the positions to a star position catalog generally pre-stored in memory. After star identification further processing determines the instantaneous spacecraft attitude or orientation with respect to stellar inertial coordinates. Comparing the instantaneous attitude with the ordered attitude generally results in differences in body coordinates pitch, roll, and/or yaw. An error signal is then generated to correct for the differences between the actual attitude and the ordered attitude.
When star-tracker operation is interrupted by a stellar body blocking the star-tracker""s field of view, e.g., the sun or moon, the non-inertial attitude reference data provided by the star-tracker is also interrupted. An alternative to non-inertial attitude reference systems are inertial attitude reference systems. Inertial reference systems are typically rate sensors that detect angular rates of change of the spacecraft""s attitude with respect to an inertial coordinate system. Inertial reference systems can be mechanical gyro devices, optical devices, such as ring laser gyros, or fiber optic gyros. However, inertial reference systems are subject to drift and must be re-calibrated periodically to eliminate the drift error introduced into the attitude data. A problem arises when the non-inertial attitude reference system is unavailable and the inertial attitude reference system is providing attitude reference data with an unknown drift error.
In accordance with one embodiment of the invention a control system for providing attitude control in spacecraft is provided. The control system comprises a primary attitude reference system, a secondary attitude reference system, and a hyper-complex number differencing system. The hyper-complex number differencing system is connectable to the primary attitude reference system and the secondary attitude reference system.
In accordance with another embodiment the invention includes a method for providing attitude control data in spacecraft. The method comprises the steps of providing non-inertial attitude reference (NAR) data derived from a non-inertial source, providing backup inertial attitude reference (IAR) data when NAR data is not available, and utilizing NAR data when available to correct for drift errors in IAR data.
In accordance with another embodiment of the invention, an improved spacecraft attitude control system is provided. The improved space control system comprising A quaternion format differencing system for Attitude Referencing control in a spacecraft, the system comprising a star-tracker reference system, wherein the star-tracker reference system comprises a first quaternion data generator, a body control module for producing orthogonal axes body control reference torques, and a quaternion comparator connectable to the star-tracker reference system and the body control module.
The invention is also directed to a computer readable medium embodying program code for providing attitude control data in spacecraft. The method comprises the steps of providing non-inertial attitude reference (NAR) data derived from a non-inertial source, providing backup inertial attitude reference (IAR) data when NAR data is not available, and utilizing NAR data when available to correct for drift errors in IAR data.